Carb cutting out at high speeds

I'm new here and built my bike which is a Kent beach cruiser with a HT motor. I'm having a problem with what I think is the carb. At high speeds the engine will cut out for a few seconds. If I let off the throttle a little it is usually fine. I thought it may be spark related so I replaced the plug and wire. Also at times when I let off the throttle the engine revs and other times it just sputters and dies. What could it be?

This sounds like the classic example of an air leak in the intake manifold or the carburetor where it attaches to the manifold. Air entering the intake downstream of the carburetor will cause a lean condition that will produce the symptoms you've described. Make sure you have a good seal where the carb slides onto the manifold. Use a good silicone gasket sealer there and at the manifold where it attaches to the cylinder. Many have had problems with the kit supplied gasket for the manifold and we suggest making a new one from quality grade gasket material available at any auto parts store. Get back to us with what you find.
Tom